It's hard to imagine that the Sun has
not always been the way it is today. Yet there was time, in the distant past,
when the Sun did not exist.

More than 5 billion years ago, there was no Sun, no Earth, no solar system at
all. There was, instead, just a huge, thin cloud of gas and dust slowly turning
and drifting through space.

Gradually, the cloud became smaller. Because of its own gravity it pulled itself
together. At the same time, it began to get hotter and denser.

By about 5 billion years ago, nearly all the cloud's gas was packed into a big,
fuzzy ball at the center of the cloud. Then a very important thing happened. Deep
inside the ball, the temperature rose high enough for fusion to start. Hydrogen
began to turn into helium, making light and heat. What had been a ball of gas
became a star: the Sun.

But there was more to come. Not all of the gas had been used up in making the
Sun. Some of it settled into a flat, pancake-shaped cloud that now circled the
newborn star. Slowly, from this cloud, the planets, moons and other members of
the Sun's family formed.

What will happen to the Sun in the future? For billions of years, it will carry
on "burning" hydrogen fuel in its core. Although the Sun uses up around 5 million
tons of hydrogen every second, it still has enough left i its core to last for
another 5 billion years or so.

When it finally does run out of fuel, though, something very odd will happen to
the Sun. It will swell up to many times its present size and become what is known
as a red giant. The outer layers of the Sun will grow to swallow up, in turn,
the planets Mercury and Venus. They may even reach out as far as Earth. Then,
the surface of our planet will be scorched, and its oceans boiled dry.

As a red giant, the Sun might be able to exist for a few more million years. During
this time it will shed matter quite quickly. The solar wind will strengthen to
a solar gale. Finally, the Sun may cast off most of its outer layers as a bright
shell of gas called a planetary nebula.

All that will be left behind is a very hot, dense core. The Sun, in fact, will
have become a white dwarf - a star no bigger than the Earth. Gradually, over many
millions of years, even this small star will cool. The Sun will end its days quietly
as a dimming ember in space.

Before this happens, human beings may have learned how to travel to other stars.
We may be able to make our home on a planet around a friendlier star. But perhaps
we will leave behind a robot probe to watch the final fate of our old neighborhood
star.